Sound system gain and equalization circuit

ABSTRACT

A multi-band equalizer for receiving and processing an audio signal includes a plurality of operational amplifiers; each for processing a different frequency band of the audio signal to produce an output signal. Each operational amplifier includes an inverting input, a non-inverting input, a resistor means coupled to the non-inverting input for receiving the audio signal and supplying it to the non-inverting input, an output, a feedback circuit coupled between the output and the inverting input for determining the upper limits of the frequency band processed by that particular operational amplifier, and an input circuit coupled between the inverting input and ground potential for determining the lower limits of the frequency band processed by that operational amplifier. A summing circuit is coupled to the outputs of all of the operational amplifiers for combining the output signals thereof to produce a resultant signal. The summing circuit includes a plurality of variable resistors, each coupled to the output of a different operational amplifier for selectively varying the magnitude of the output signal from the respective operational amplifiers.

This application is a continuation of application Ser. No. 08/054,036,filed Apr. 28, 1993 now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to an equalizer circuit and more particularly toa gain and equalization circuit for use in reducing distortion and otheranomalies, and improving "clarity" in a variety of currently availablesound systems.

Sound generation, recording and reproduction systems may take a varietyof forms and perform a variety of functions, all relating, of course, toprocessing sound signals, with the common objective being to ultimatelyreproduce as accurately as possible the sound originally created orrecorded or even "enhance" it. Such systems include, among others,public address systems and similar systems which utilize microphones andspeakers, radio and television broadcast systems, radio and televisionreceivers, tape recorders and disk recorders and players, home, auto andportable stereo systems, and recording studio systems. In all suchsystems, the sound is converted to electrical audio signals representingthe sound, processed in some way, and then either reproduced,transmitted to other locations or recorded. At the various stages ofgenerating the sound and processing the audio signals, there is a chancethat either noise will be introduced to mask the true signals or thesignals will be distorted (undesired change in signal waveform) in sucha way that it is difficult to accurately reproduce the sound. Such noiseand/or distortion may arise in the sound source itself, for example,instruments, voices, etc., in the room or studio acoustic configuration,in microphones which pick up the sound and convert it to electricalaudio signals, in audio amplifiers and other audio signal processingcomponents, in recording equipment and recording media, in speakersystems, and in audio signal transmitting equipment.

Ideally, all noise would be removed from (or not allowed to initiallyinfluence) the audio signal, and all processing of the audio signalwould take place free from distortion, e.g., amplification would occurequally and uniformly over the entire audio signal frequency band (audiospectrum). However, achieving an essentially undistorted resultant audiosignal has not been possible; rather in the course of reproducing anaudio signal and otherwise processing such a signal, distortion of someform (phase distortion, frequency distortion, harmonic distortion,intermodulation distortion and the addition of noise) is inevitablyintroduced. Distortion, which is frequency dependent, means that thesignal being processed is treated differently, e.g. amplified or phaseshifted by different amounts, at the different frequencies contained inthe signal. Such distortion prevents the accurate reproduction of theoriginal sound transmitted, recorded, or produced.

In an attempt to reduce, to the extent possible, distortion and otherundesirable deficiencies produced by room acoustics, microphones,loudspeakers, recorders, and other audio signal producing and processingcomponents, what are called "equalizers" are provided. Equalizers effector introduce a kind of controlled distortion of the frequency responsewhich is ideally flat, for the purpose of offsetting or cancelling thedistortion introduced during signal origination production andprocessing. Equalizers, in effect, alter the frequency response of anaudio system in some desired manner. Initially equalizers wereconstructed of passive components, to provide attenuation or cuts atcertain frequencies. Later designs were usually constructed with activecomponents, typically vacuum tube circuits and operational amplifiers.

Among the more well known equalizers in use today is the so-calledgraphic equalizer which is incorporated into many professional, home andautomobile sound systems. The graphic equalizer is generally constructedso that the console and controls present the appearance of a graphicdisplay of the frequency response being developed by the equalizer,e.g., which bands of the audio signal are boosted and which are cut.

In another type of equalizer, known as the parametric equalizer, threeparameters of equalization, including frequency selection, boost or cut,and bandwidth control, are all independently variable.

More elaborate studio equalizers are utilized in recording, broadcastand television studios and these consist basically of a parallel bank ofband-pass filters in which the center frequencies of the filters areseparated by some finite amount such as an octave or fraction thereof,typically one-third. The gain or attenuation of each filter isseparately adjustable, the result of which is an overall frequencyresponse which can be continuously set across the entire audio frequencyrange.

The inability of presently available equalizers to perform high quality"equalization" is indicated by a recent comment in MIX, December 1992,page 22 which reads:

"There has always been a serious problem with EQ I refer, of course, toour old friend Mr. Analog EQ It has never worked right and it neverwill.

While Analog EQ will change the response characteristic of a circuit,thereby increasing or decreasing the amount of passed energy in a givenfrequency band, the price is terribly high.

EQ made your tracks slur, soften up, come apart. It didn't take you longto learn that EQ was a tradeoff. You used as much as you could beforethe side effects did more damage than the EQ did good.

All this is due to phase shift. More EQ, more phase shift; that's howit's done| Until true zero-coefficient FIR digital EQ came alongrecently, equalizing without phase shift was just a dream."

Digital equalization is mentioned in the above-quoted comment as a meansof achieving equalization without phase shift and this may or may not betrue, but in any case, digital equalization is very costly, requiringanalog-to-digital conversion circuitry, digital processing circuitry,and digital-to-analog conversion circuitry, all of which tend to modifyor distort the signal in some way.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a new and improved equalizercircuit for processing and performing equalization on audio signals.

It is a further object of the invention to provide such a circuit whichis capable of selectively providing gain as well as equalization to anaudio input signal.

It is another object of the invention to provide such a circuit foreffectively reducing distortion, thereby improving clarity in currentlyavailable sound systems.

It is still another object of the invention to provide an analogequalizer which is simple in design, relatively inexpensive and yeteffective in performing equalization without significant phase shift.

It is an additional object of the invention to provide such a circuitcapable of accurately reproducing an audio square wave.

It is also an object of the invention to provide such a circuit whichmay be implemented as part of an audio preamplifier and in otherenvironments requiring equalization.

It is a further object of the invention to provide a gain andequalization circuit having a substantially undistorted (flat) frequencyresponse.

The above and other objects of the invention are realized in a specificillustrative embodiment of a gain and equalization circuit forprocessing received audio signal and including a plurality ofoperational amplifiers, each for processing and selectively amplifying adifferent band of frequencies of the audio signal to produce an outputsignal. Each operational amplifier includes an inverting input, anon-inverting input for receiving the audio signal, an output, a highpass filter network coupled between the inverting input and groundpotential for determining the lower end of the frequency band processedby the operational amplifier, and a low pass filter network coupledbetween the output and the inverting input for determining the upper endof the frequency band processed by the operational amplifier. The gainand equalization circuit further includes a summing circuit coupled tothe outputs of the operational amplifiers for selectively attenuatingselected output signals and for combining the signals to produce aresultant combined signal covering the audio spectrum or desired portionthereof.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the inventionwill become apparent from a consideration of the following detaileddescription presented in connection with the accompanying drawings inwhich:

FIG. 1 shows a schematic of a gain and equalization circuit made inaccordance with the principles of the present invention; and

FIG. 2 shows individual waveforms produced by the operational amplifiercircuits of FIG. 1, and a resultant waveform produced by combining theindividual waveforms.

DETAILED DESCRIPTION

Referring to FIG. 1, there is shown an illustrative embodiment of anequalizer circuit made in accordance with the present invention toinclude a plurality of operational amplifiers 4, with associatedfeedback circuitry 8 and input circuitry 12, connected in parallel withone another. An audio signal input terminal 14 is coupled by way of aresistor divider 16 and 17, and resistors 18 to respective non-invertinginputs of the operational amplifiers 4. A plurality of input circuits 12composed of series connections, of a capacitor 20 and a resistor 24, arecoupled between ground potential and the inverting input of a respectiveoperational amplifier 4. A plurality of feedback circuits each composedof a parallel connection of a capacitor 28 and resistor 32, couples theoutput of a respective operational amplifier 4 to the inverting inputthereof, as shown.

The output of each operational amplifier 4 is coupled to a summingcircuit 40 via a plurality of variable resistors 36, each coupled to theoutput of a different operational amplifier 4. The summing circuit 40includes an operational amplifier 44 whose non-inverting input iscoupled to ground and whose output is coupled by way of a parallelconnection of a capacitor 48 and a resistor 52 to the inverting input ofthe amplifier. Each of the variable resistors 36 is likewise coupled tothe inverting input of the operational amplifier 44.

The output of the summing circuit 40 is coupled to an output loadisolation circuit 60 which is composed of a capacitor 64, a resistor 68and inductor 72 coupled in parallel between the capacitor 64 and anoutput terminal 76, and a resistor 80 coupled from ground potential tothe node between capacitor 64, resistor 68 and inductor 72.

Each of the operational amplifiers 4, with the associated input andfeedback circuitry, acts as a filter to pass a different frequency bandof the input audio signal. For example, if a six band equalizer weredesired, and therefore six operational amplifiers were provided, thefrequency centers for the six bands could illustratively be, but notlimited to, 10 Hz, 40 Hz, 160 Hz, 640 Hz, 2560 Hz. and 10240 Hz with theskirts varying ≈11/2 dB at one octave from the respective centerfrequency, ≈6 dB at two octaves, ≈10 dB at three octaves, and ≈131/2 dBat four octaves. The values of the feedback R/C network of capacitors 28and resistors 32, and the input R/C network of capacitors 20 andresistors 24 are selected to provide the respective center frequenciesof the bands in question. The feedback R/C network 8 of each operationalamplifier forms a low pass filter to determine the high end roll-off orcut-off of the bands in question, while the input RIC network 12 of eachoperational amplifier forms a high pass filter to determine the low endroll-off of the band. With the configuration of FIG. 1, the same valuecapacitors can be used for capacitors 20, and same value capacitors canbe used for capacitors 28, with different value resistors being requiredto provide the desired operating characteristics. For example, toobtain, the center frequencies identified above for a six bandequalizer, suitable values for the capacitors and resistors could be:

resistor 24a=8 ohms

resistor 24b=32 ohms

resistor 24c=128 ohms

resistor 24d=512 ohms

resistor 24e=2048 ohms

resistor 24f=8192 ohms

resistor 32a=80 ohms

resistor 32b=320 ohms

resistor 32c=1280 ohms

resistor 32d=5120 ohms

resistor 32e=20,480 ohms

resistor 32f=81,920 ohms

Exemplary values for the other circuit components are:

resistors 18=1.0k ohms

variable resistors 36=from 1.1k ohms to 101.1k ohms

resistor 52=11.1k ohms

capacitor 48=22 pf

capacitor 64=470 μf

resistor 80=10k ohms

resistor 68=47 ohms

inductor 72=100 μHenrys

The operational amplifiers 4 of FIG. 1 might illustratively be, but isnot limited to, model NE5532 AN amplifiers, made by Signetics.

With the circuit configuration described, phase distortion (phase shift)is reduced, with the center frequencies having substantially no phaseshift when measured at the output of each band, and with only marginalphase shift occurring toward the high and low ends of the band. Theresultant signal (from summing all band contributions) is a highclarity, substantially distortion free audio signal.

FIG. 2 shows output waveforms of several of the adjacent operationalamplifiers of FIG. 1, together with the resulting waveform obtained fromcombining or summing the several waveforms, to indicate the flatfrequency response achievable.

The resistor divider 16 is provided to attenuate or reduce the inputlevel of the audio signal supplied to the input terminal 14 if themagnitude of such a signal is too great, such as when the audio signalbeing received is from an audio line amplifier. In the course ofperforming equalization by the operational amplifiers 4 and associatedcircuitry, the audio signal is again boosted to the desired level, butwith the ability to tailor the frequency response.

If no attenuation of the audio input signal is necessary, then resistordivider 16 would be eliminated from the FIG. 1 circuit. This would bethe case if, for example, the equalizer circuit of FIG. 1 were utilizedas a combination equalizer/preamplifier in a microphone/speaker system.In such case, the circuitry of FIG. 1 would simply be substituted forthe conventional pre-amplifier in the microphone/speaker system and theoperational amplifiers 4 with associated circuitry, would provide boththe desired amplification or "pre-amplification" and equalization of thesignals received from the microphone.

Resistors 18 are provided to stabilize the operation and processing ofthe input signal by the operational amplifiers 4.

Variable resistors 36 are provided to either selectively allow gain inor to attenuate the respective band being supplied thereto.Advantageously, eleven position variable resistor switches are provided,each to provide five levels of gain and five levels of attenuation, witha central position being one in which no change in signal level occurs.In this manner, the contribution of each operational amplifier to theresulting "equalized" signal can be determined by manual adjustment ofthe variable resistors 36. The contributions from each operationalamplifier are combined by the summing circuit 40 and then passed via theisolator circuit 60 to the output terminal 76.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention and the appended claims are intendedto cover such modifications and arrangements.

What is claimed is:
 1. A voltage gain and equalization circuit forprocessing a received audio signal, comprising:a plurality ofoperational amplifier means, each for processing and selectivelyamplifying a different frequency band of the audio signal to produce anoutput signal with generally no phase shift, and each includinganinverting input, a non-inverting input for receiving the audio signal,an output, high pass filter means coupled between the inverting inputand ground for determining the lower end of the frequency band processedby said each operational amplifier means, low pass filter means coupledbetween the output and the inverting input for determining the upper endof the frequency band processed by said each operational amplifiermeans, and summing means coupled to the outputs of each operationalamplifier means for combining the output signals, each output signalpassing a different frequency band of the audio signal thereof, toproduce a resultant signal.
 2. A voltage gain and equalization circuitas in claim 1 further including an isolation network for coupling thecombining means to a load.
 3. A voltage gain and equalization circuit asin claim 1 further including a plurality of variable resistance means,each coupled between the output of a respective operational amplifiermeans and the combining means, for selectively varying the magnitude ofthe output signal from said respective operational amplifier means.
 4. Avoltage gain and equalization circuit as in claim 1 wherein eachoperational amplifier means includes resistance means coupled to thenon-inverting input of said each operational amplifier means.
 5. Avoltage gain and equalization circuit as in claim 4 wherein each highpass filter means comprises a series connection of a first capacitor anda first resistor coupled between the inverting input and ground, whereinthe capacitance of the first capacitor and resistance of the firstresistor are selected to roll off the low end of a frequency banddifferent from those of the other operational amplifier means.
 6. Avoltage gain and equalization circuit as in claim 5 wherein each lowpass filter means comprises a parallel connection of a second capacitorand a second resistor, wherein the capacitance of the second capacitorand resistance of the second resistor are selected to roll off the highend of a frequency band different from those of the other operationalamplifier means.
 7. A voltage gain and equalization circuit as in claim6 wherein the separation between frequency bands processed by theplurality of operational amplifier means is about a factor of four.
 8. Avoltage gain and equalization circuit as in claim 6 wherein there aresix operational amplifier means for processing respective ones of sixdifferent bands of the audio signal, where the center frequencies of therespective bands are 10 Hz, 40 Hz, 160 Hz, 640 Hz, 2560 Hz and 10240 Hz,with the skirts varying about 11/2 dB at one octave from the respectivecenter frequency, about 6 dB at two octaves, about 10 dB at threeoctaves and about 131/2 dB at four octaves.
 9. A voltage gain andequalization circuit as in claim 6 wherein the capacitance values of allthe first capacitors are substantially the same, and wherein thecapacitance values of all the second capacitors are substantially thesame.
 10. A voltage gain and equalization circuit as in claim 1 whereinthe output of the plurality of operational amplifier means is coupled tothe non-inverting input of the combining means.
 11. A multiple-bandequalizer for receiving and processing an audio signal comprisingaplurality of operational amplifier means, each for processing adifferent frequency band of the audio signal to produce an outputsignal, and each includingan inverting input, a non-inverting input, anoutput, feedback circuit means- coupled between the output and theinverting input for determining the upper limits of the frequency bandprocessed by said each operational amplifier means, and input circuitmeans coupled between the inverting input and ground potential fordetermining the lower limits of the frequency band processed by saideach operational amplifier means, summing circuit means coupled to theoutputs of each operation amplifier means for combining the outputsignals thereof to produce a resultant signal.
 12. An equalizer as inclaim 11 further including a plurality of variable resistance means,each coupled to the output of a respective operational amplifier meansfor selectively varying the magnitude of the output signal from saidrespective operational amplifier means.
 13. An equalizer as in claim 12wherein each feedback circuit means includes a parallel connection of acapacitor and a resistor, and wherein each input circuit means includesa series connection of a capacitor and a resistor.
 14. A voltage gainand equalization circuit as in claim 11 wherein the output of theplurality of operational amplifier means is coupled to the non-invertinginput of the summing circuit means.